Added a UserPolicySigninService for iOS.

This is just a frontend to UserPolicySigninServiceBase that uses blocks
instead of base::Callbacks, for ease of integration into the Chrome iOS UI.

BUG=275292
R=dconnelly@chromium.org

Review URL: https://codereview.chromium.org/146833009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247241 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247241